<!doctype html>
<html>
<head>
<meta charset='UTF-8'><meta name='viewport' content='width=device-width initial-scale=1'>
<title>9-范式</title><link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; }
html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
body { margin: 0px; padding: 0px; height: auto; bottom: 0px; top: 0px; left: 0px; right: 0px; font-size: 1rem; line-height: 1.42857; overflow-x: hidden; background: inherit; tab-size: 4; }
iframe { margin: auto; }
a.url { word-break: break-all; }
a:active, a:hover { outline: 0px; }
.in-text-selection, ::selection { text-shadow: none; background: var(--select-text-bg-color); color: var(--select-text-font-color); }
#write { margin: 0px auto; height: auto; width: inherit; word-break: normal; overflow-wrap: break-word; position: relative; white-space: normal; overflow-x: visible; padding-top: 40px; }
#write.first-line-indent p { text-indent: 2em; }
#write.first-line-indent li p, #write.first-line-indent p * { text-indent: 0px; }
#write.first-line-indent li { margin-left: 2em; }
.for-image #write { padding-left: 8px; padding-right: 8px; }
body.typora-export { padding-left: 30px; padding-right: 30px; }
.typora-export .footnote-line, .typora-export li, .typora-export p { white-space: pre-wrap; }
@media screen and (max-width: 500px) {
  body.typora-export { padding-left: 0px; padding-right: 0px; }
  #write { padding-left: 20px; padding-right: 20px; }
  .CodeMirror-sizer { margin-left: 0px !important; }
  .CodeMirror-gutters { display: none !important; }
}
#write li > figure:last-child { margin-bottom: 0.5rem; }
#write ol, #write ul { position: relative; }
img { max-width: 100%; vertical-align: middle; }
button, input, select, textarea { color: inherit; font: inherit; }
input[type="checkbox"], input[type="radio"] { line-height: normal; padding: 0px; }
*, ::after, ::before { box-sizing: border-box; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p, #write pre { width: inherit; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p { position: relative; }
p { line-height: inherit; }
h1, h2, h3, h4, h5, h6 { break-after: avoid-page; break-inside: avoid; orphans: 2; }
p { orphans: 4; }
h1 { font-size: 2rem; }
h2 { font-size: 1.8rem; }
h3 { font-size: 1.6rem; }
h4 { font-size: 1.4rem; }
h5 { font-size: 1.2rem; }
h6 { font-size: 1rem; }
.md-math-block, .md-rawblock, h1, h2, h3, h4, h5, h6, p { margin-top: 1rem; margin-bottom: 1rem; }
.hidden { display: none; }
.md-blockmeta { color: rgb(204, 204, 204); font-weight: 700; font-style: italic; }
a { cursor: pointer; }
sup.md-footnote { padding: 2px 4px; background-color: rgba(238, 238, 238, 0.7); color: rgb(85, 85, 85); border-radius: 4px; cursor: pointer; }
sup.md-footnote a, sup.md-footnote a:hover { color: inherit; text-transform: inherit; text-decoration: inherit; }
#write input[type="checkbox"] { cursor: pointer; width: inherit; height: inherit; }
figure { overflow-x: auto; margin: 1.2em 0px; max-width: calc(100% + 16px); padding: 0px; }
figure > table { margin: 0px !important; }
tr { break-inside: avoid; break-after: auto; }
thead { display: table-header-group; }
table { border-collapse: collapse; border-spacing: 0px; width: 100%; overflow: auto; break-inside: auto; text-align: left; }
table.md-table td { min-width: 32px; }
.CodeMirror-gutters { border-right: 0px; background-color: inherit; }
.CodeMirror-linenumber { user-select: none; }
.CodeMirror { text-align: left; }
.CodeMirror-placeholder { opacity: 0.3; }
.CodeMirror pre { padding: 0px 4px; }
.CodeMirror-lines { padding: 0px; }
div.hr:focus { cursor: none; }
#write pre { white-space: pre-wrap; }
#write.fences-no-line-wrapping pre { white-space: pre; }
#write pre.ty-contain-cm { white-space: normal; }
.CodeMirror-gutters { margin-right: 4px; }
.md-fences { font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; overflow: visible; white-space: pre; background: inherit; position: relative !important; }
.md-diagram-panel { width: 100%; margin-top: 10px; text-align: center; padding-top: 0px; padding-bottom: 8px; overflow-x: auto; }
#write .md-fences.mock-cm { white-space: pre-wrap; }
.md-fences.md-fences-with-lineno { padding-left: 0px; }
#write.fences-no-line-wrapping .md-fences.mock-cm { white-space: pre; overflow-x: auto; }
.md-fences.mock-cm.md-fences-with-lineno { padding-left: 8px; }
.CodeMirror-line, twitterwidget { break-inside: avoid; }
.footnotes { opacity: 0.8; font-size: 0.9rem; margin-top: 1em; margin-bottom: 1em; }
.footnotes + .footnotes { margin-top: 0px; }
.md-reset { margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: top; background: 0px 0px; text-decoration: none; text-shadow: none; float: none; position: static; width: auto; height: auto; white-space: nowrap; cursor: inherit; -webkit-tap-highlight-color: transparent; line-height: normal; font-weight: 400; text-align: left; box-sizing: content-box; direction: ltr; }
li div { padding-top: 0px; }
blockquote { margin: 1rem 0px; }
li .mathjax-block, li p { margin: 0.5rem 0px; }
li { margin: 0px; position: relative; }
blockquote > :last-child { margin-bottom: 0px; }
blockquote > :first-child, li > :first-child { margin-top: 0px; }
.footnotes-area { color: rgb(136, 136, 136); margin-top: 0.714rem; padding-bottom: 0.143rem; white-space: normal; }
#write .footnote-line { white-space: pre-wrap; }
@media print {
  body, html { border: 1px solid transparent; height: 99%; break-after: avoid; break-before: avoid; }
  #write { margin-top: 0px; padding-top: 0px; border-color: transparent !important; }
  .typora-export * { -webkit-print-color-adjust: exact; }
  html.blink-to-pdf { font-size: 13px; }
  .typora-export #write { padding-left: 32px; padding-right: 32px; padding-bottom: 0px; break-after: avoid; }
  .typora-export #write::after { height: 0px; }
}
.footnote-line { margin-top: 0.714em; font-size: 0.7em; }
a img, img a { cursor: pointer; }
pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
p > .md-image:only-child { display: inline-block; width: 100%; }
#write .MathJax_Display { margin: 0.8em 0px 0px; }
.md-math-block { width: 100%; }
.md-math-block:not(:empty)::after { display: none; }
[contenteditable="true"]:active, [contenteditable="true"]:focus { outline: 0px; box-shadow: none; }
.md-task-list-item { position: relative; list-style-type: none; }
.task-list-item.md-task-list-item { padding-left: 0px; }
.md-task-list-item > input { position: absolute; top: 0px; left: 0px; margin-left: -1.2em; margin-top: calc(1em - 10px); border: none; }
.math { font-size: 1rem; }
.md-toc { min-height: 3.58rem; position: relative; font-size: 0.9rem; border-radius: 10px; }
.md-toc-content { position: relative; margin-left: 0px; }
.md-toc-content::after, .md-toc::after { display: none; }
.md-toc-item { display: block; color: rgb(65, 131, 196); }
.md-toc-item a { text-decoration: none; }
.md-toc-inner:hover { text-decoration: underline; }
.md-toc-inner { display: inline-block; cursor: pointer; }
.md-toc-h1 .md-toc-inner { margin-left: 0px; font-weight: 700; }
.md-toc-h2 .md-toc-inner { margin-left: 2em; }
.md-toc-h3 .md-toc-inner { margin-left: 4em; }
.md-toc-h4 .md-toc-inner { margin-left: 6em; }
.md-toc-h5 .md-toc-inner { margin-left: 8em; }
.md-toc-h6 .md-toc-inner { margin-left: 10em; }
@media screen and (max-width: 48em) {
  .md-toc-h3 .md-toc-inner { margin-left: 3.5em; }
  .md-toc-h4 .md-toc-inner { margin-left: 5em; }
  .md-toc-h5 .md-toc-inner { margin-left: 6.5em; }
  .md-toc-h6 .md-toc-inner { margin-left: 8em; }
}
a.md-toc-inner { font-size: inherit; font-style: inherit; font-weight: inherit; line-height: inherit; }
.footnote-line a:not(.reversefootnote) { color: inherit; }
.md-attr { display: none; }
.md-fn-count::after { content: "."; }
code, pre, samp, tt { font-family: var(--monospace); }
kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
.md-comment { color: rgb(162, 127, 3); opacity: 0.8; font-family: var(--monospace); }
code { text-align: left; vertical-align: initial; }
a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
.md-inline-math .MathJax_SVG .noError { display: none !important; }
.html-for-mac .inline-math-svg .MathJax_SVG { vertical-align: 0.2px; }
.md-math-block .MathJax_SVG_Display { text-align: center; margin: 0px; position: relative; text-indent: 0px; max-width: none; max-height: none; min-height: 0px; min-width: 100%; width: auto; overflow-y: hidden; display: block !important; }
.MathJax_SVG_Display, .md-inline-math .MathJax_SVG_Display { width: auto; margin: inherit; display: inline-block !important; }
.MathJax_SVG .MJX-monospace { font-family: var(--monospace); }
.MathJax_SVG .MJX-sans-serif { font-family: sans-serif; }
.MathJax_SVG { display: inline; font-style: normal; font-weight: 400; line-height: normal; zoom: 90%; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; }
.MathJax_SVG * { transition: none 0s ease 0s; }
.MathJax_SVG_Display svg { vertical-align: middle !important; margin-bottom: 0px !important; margin-top: 0px !important; }
.os-windows.monocolor-emoji .md-emoji { font-family: "Segoe UI Symbol", sans-serif; }
.md-diagram-panel > svg { max-width: 100%; }
[lang="mermaid"] svg, [lang="flow"] svg { max-width: 100%; height: auto; }
[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
.highlight td, .highlight tr { border: 0px; }


.CodeMirror { height: auto; }
.CodeMirror.cm-s-inner { background: inherit; }
.CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
.CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
.CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
.CodeMirror-linenumber { padding: 0px 3px 0px 5px; text-align: right; color: rgb(153, 153, 153); }
.cm-s-inner .cm-keyword { color: rgb(119, 0, 136); }
.cm-s-inner .cm-atom, .cm-s-inner.cm-atom { color: rgb(34, 17, 153); }
.cm-s-inner .cm-number { color: rgb(17, 102, 68); }
.cm-s-inner .cm-def { color: rgb(0, 0, 255); }
.cm-s-inner .cm-variable { color: rgb(0, 0, 0); }
.cm-s-inner .cm-variable-2 { color: rgb(0, 85, 170); }
.cm-s-inner .cm-variable-3 { color: rgb(0, 136, 85); }
.cm-s-inner .cm-string { color: rgb(170, 17, 17); }
.cm-s-inner .cm-property { color: rgb(0, 0, 0); }
.cm-s-inner .cm-operator { color: rgb(152, 26, 26); }
.cm-s-inner .cm-comment, .cm-s-inner.cm-comment { color: rgb(170, 85, 0); }
.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
.cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
.cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
.cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
.cm-s-inner .cm-attribute { color: rgb(0, 0, 204); }
.cm-s-inner .cm-header, .cm-s-inner.cm-header { color: rgb(0, 0, 255); }
.cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
.cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
.cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
.cm-negative { color: rgb(221, 68, 68); }
.cm-positive { color: rgb(34, 153, 34); }
.cm-header, .cm-strong { font-weight: 700; }
.cm-del { text-decoration: line-through; }
.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
.cm-error { color: red; }
.cm-invalidchar { color: red; }
.cm-constant { color: rgb(38, 139, 210); }
.cm-defined { color: rgb(181, 137, 0); }
div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
.cm-s-inner .CodeMirror-activeline-background { background: inherit; }
.CodeMirror { position: relative; overflow: hidden; }
.CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
.CodeMirror-sizer { position: relative; }
.CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; }
.CodeMirror-vscrollbar { right: 0px; top: 0px; overflow: hidden; }
.CodeMirror-hscrollbar { bottom: 0px; left: 0px; overflow: hidden; }
.CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
.CodeMirror-gutter-filler { left: 0px; bottom: 0px; }
.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
.CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
.CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
.CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
.CodeMirror-linebackground { position: absolute; left: 0px; right: 0px; top: 0px; bottom: 0px; z-index: 0; }
.CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
.CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
.CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
.CodeMirror-measure pre { position: static; }
.CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
.CodeMirror div.CodeMirror-cursor { visibility: hidden; }
.CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
.cm-searching { background: rgba(255, 255, 0, 0.4); }
@media print {
  .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
}


:root { --side-bar-bg-color: #fafafa; --control-text-color: #777; }
html { font-size: 16px; }
body { font-family: "Open Sans", "Clear Sans", "Helvetica Neue", Helvetica, Arial, sans-serif; color: rgb(51, 51, 51); line-height: 1.6; }
#write { max-width: 860px; margin: 0px auto; padding: 30px 30px 100px; }
#write > ul:first-child, #write > ol:first-child { margin-top: 30px; }
a { color: rgb(65, 131, 196); }
h1, h2, h3, h4, h5, h6 { position: relative; margin-top: 1rem; margin-bottom: 1rem; font-weight: bold; line-height: 1.4; cursor: text; }
h1:hover a.anchor, h2:hover a.anchor, h3:hover a.anchor, h4:hover a.anchor, h5:hover a.anchor, h6:hover a.anchor { text-decoration: none; }
h1 tt, h1 code { font-size: inherit; }
h2 tt, h2 code { font-size: inherit; }
h3 tt, h3 code { font-size: inherit; }
h4 tt, h4 code { font-size: inherit; }
h5 tt, h5 code { font-size: inherit; }
h6 tt, h6 code { font-size: inherit; }
h1 { padding-bottom: 0.3em; font-size: 2.25em; line-height: 1.2; border-bottom: 1px solid rgb(238, 238, 238); }
h2 { padding-bottom: 0.3em; font-size: 1.75em; line-height: 1.225; border-bottom: 1px solid rgb(238, 238, 238); }
h3 { font-size: 1.5em; line-height: 1.43; }
h4 { font-size: 1.25em; }
h5 { font-size: 1em; }
h6 { font-size: 1em; color: rgb(119, 119, 119); }
p, blockquote, ul, ol, dl, table { margin: 0.8em 0px; }
li > ol, li > ul { margin: 0px; }
hr { height: 2px; padding: 0px; margin: 16px 0px; background-color: rgb(231, 231, 231); border: 0px none; overflow: hidden; box-sizing: content-box; }
li p.first { display: inline-block; }
ul, ol { padding-left: 30px; }
ul:first-child, ol:first-child { margin-top: 0px; }
ul:last-child, ol:last-child { margin-bottom: 0px; }
blockquote { border-left: 4px solid rgb(223, 226, 229); padding: 0px 15px; color: rgb(119, 119, 119); }
blockquote blockquote { padding-right: 0px; }
table { padding: 0px; word-break: initial; }
table tr { border-top: 1px solid rgb(223, 226, 229); margin: 0px; padding: 0px; }
table tr:nth-child(2n), thead { background-color: rgb(248, 248, 248); }
table tr th { font-weight: bold; border-width: 1px 1px 0px; border-top-style: solid; border-right-style: solid; border-left-style: solid; border-top-color: rgb(223, 226, 229); border-right-color: rgb(223, 226, 229); border-left-color: rgb(223, 226, 229); border-image: initial; border-bottom-style: initial; border-bottom-color: initial; margin: 0px; padding: 6px 13px; }
table tr td { border: 1px solid rgb(223, 226, 229); margin: 0px; padding: 6px 13px; }
table tr th:first-child, table tr td:first-child { margin-top: 0px; }
table tr th:last-child, table tr td:last-child { margin-bottom: 0px; }
.CodeMirror-lines { padding-left: 4px; }
.code-tooltip { box-shadow: rgba(0, 28, 36, 0.3) 0px 1px 1px 0px; border-top: 1px solid rgb(238, 242, 242); }
.md-fences, code, tt { border: 1px solid rgb(231, 234, 237); background-color: rgb(248, 248, 248); border-radius: 3px; padding: 2px 4px 0px; font-size: 0.9em; }
code { background-color: rgb(243, 244, 244); padding: 0px 2px; }
.md-fences { margin-bottom: 15px; margin-top: 15px; padding-top: 8px; padding-bottom: 6px; }
.md-task-list-item > input { margin-left: -1.3em; }
@media print {
  html { font-size: 13px; }
  table, pre { break-inside: avoid; }
  pre { overflow-wrap: break-word; }
}
.md-fences { background-color: rgb(248, 248, 248); }
#write pre.md-meta-block { padding: 1rem; font-size: 85%; line-height: 1.45; background-color: rgb(247, 247, 247); border: 0px; border-radius: 3px; color: rgb(119, 119, 119); margin-top: 0px !important; }
.mathjax-block > .code-tooltip { bottom: 0.375rem; }
.md-mathjax-midline { background: rgb(250, 250, 250); }
#write > h3.md-focus::before { left: -1.5625rem; top: 0.375rem; }
#write > h4.md-focus::before { left: -1.5625rem; top: 0.285714rem; }
#write > h5.md-focus::before { left: -1.5625rem; top: 0.285714rem; }
#write > h6.md-focus::before { left: -1.5625rem; top: 0.285714rem; }
.md-image > .md-meta { border-radius: 3px; padding: 2px 0px 0px 4px; font-size: 0.9em; color: inherit; }
.md-tag { color: rgb(167, 167, 167); opacity: 1; }
.md-toc { margin-top: 20px; padding-bottom: 20px; }
.sidebar-tabs { border-bottom: none; }
#typora-quick-open { border: 1px solid rgb(221, 221, 221); background-color: rgb(248, 248, 248); }
#typora-quick-open-item { background-color: rgb(250, 250, 250); border-color: rgb(254, 254, 254) rgb(229, 229, 229) rgb(229, 229, 229) rgb(238, 238, 238); border-style: solid; border-width: 1px; }
.on-focus-mode blockquote { border-left-color: rgba(85, 85, 85, 0.12); }
header, .context-menu, .megamenu-content, footer { font-family: "Segoe UI", Arial, sans-serif; }
.file-node-content:hover .file-node-icon, .file-node-content:hover .file-node-open-state { visibility: visible; }
.mac-seamless-mode #typora-sidebar { background-color: var(--side-bar-bg-color); }
.md-lang { color: rgb(180, 101, 77); }
.html-for-mac .context-menu { --item-hover-bg-color: #E6F0FE; }
#md-notification .btn { border: 0px; }
.dropdown-menu .divider { border-color: rgb(229, 229, 229); }
.ty-preferences .window-content { background-color: rgb(250, 250, 250); }
.ty-preferences .nav-group-item.active { color: white; background: rgb(153, 153, 153); }


</style>
</head>
<body class='typora-export os-windows' >
<div  id='write'  class = 'is-node'><h3><a name="示例表数据" class="md-header-anchor"></a><span>示例表数据</span></h3><p><span>假设有一个名为</span><code>employee</code><span>的员工表，它有九个属性：</span><code>id</code><span>(员工编号)、</span><code>name</code><span>(员工名称)、</span><code>mobile</code><span>(电话)、</span><code>zip</code><span>(邮编)、</span><code>province</code><span>(省份)、</span><code>city</code><span>(城市)、</span><code>district</code><span>(区县)、</span><code>deptNo</code><span>(所属部门编号)、</span><code>deptName</code><span>(所属部门名称)、表总数据如下：</span></p><figure><table><thead><tr><th><span>id</span></th><th><span>name</span></th><th><span>mobile</span></th><th><span>zip</span></th><th><span>province</span></th><th><span>city</span></th><th><span>district</span></th><th><span>deptNo</span></th><th><span>deptName</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000001</span><br><span>13910000002</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D1</span></td><td><span>部门1</span></td></tr><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000001</span><br><span>13910000002</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D2</span></td><td><span>部门2</span></td></tr><tr><td><span>102</span></td><td><span>李四</span></td><td><span>13910000003</span></td><td><span>200001</span></td><td><span>上海</span></td><td><span>上海</span></td><td><span>静安区</span></td><td><span>D3</span></td><td><span>部门3</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>13910000004</span></td><td><span>510001</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td><td><span>D4</span></td><td><span>部门4</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>13910000004</span></td><td><span>510001</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td><td><span>D5</span></td><td><span>部门 5</span></td></tr></tbody></table></figure><p><span>由于此员工表是非规范化的，我们将面对如下的问题。</span></p><blockquote><ul><li><strong><span>修改异常</span></strong><span>：上表中张三有两条记录，因为他隶属于两个部门。如果我们要修改张三的地址，必修修改两行记录。假如一个部门得到了张三的新地址并进行了更新，而另一个部门没有，那么此时张三在表中会存在两个不同的地址，导致了数据不一致</span></li><li><strong><span>新增异常：</span></strong><span>假如一个新员工假如公司，他正处于入职培训阶段，还没有被正式分配到某个部门，如果</span><code>deptNo</code><span>字段不允许为空，我们就无法向</span><code>employee</code><span>表中新增该员工的数据。</span></li><li><strong><span>删除异常：</span></strong><span>假设公司撤销了D3部门，那么在删除</span><code>deptNo</code><span>为D3的行时，会将李四的信息也一并删除。因为他隶属于D3这一部门。</span></li></ul></blockquote><h3><a name="第一范式1nf" class="md-header-anchor"></a><span>第一范式(1NF)</span></h3><blockquote><p><strong><span>表中的列只能含有原子性(不可再分)的值。</span></strong></p></blockquote><p><span>表中的张三有两个手机号存储在mobile列中，违反了 1NF 规则。为了使表满足 1NF，数据应该修改如下：</span></p><figure><table><thead><tr><th><span>id</span></th><th><span>name</span></th><th><span>mobile</span></th><th><span>zip</span></th><th><span>province</span></th><th><span>city</span></th><th><span>district</span></th><th><span>deptNo</span></th><th><span>deptName</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000001</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D1</span></td><td><span>部门1</span></td></tr><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000002</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D1</span></td><td><span>部门1</span></td></tr><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000001</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D2</span></td><td><span>部门2</span></td></tr><tr><td><span>101</span></td><td><span>张三</span></td><td><span>13910000002</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td><td><span>D2</span></td><td><span>部门2</span></td></tr><tr><td><span>102</span></td><td><span>李四</span></td><td><span>13910000003</span></td><td><span>200001</span></td><td><span>上海</span></td><td><span>上海</span></td><td><span>静安区</span></td><td><span>D3</span></td><td><span>部门3</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>13910000004</span></td><td><span>510001</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td><td><span>D4</span></td><td><span>部门4</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>13910000004</span></td><td><span>510001</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td><td><span>D5</span></td><td><span>部门 5</span></td></tr></tbody></table></figure><h3><a name="第二范式2nf" class="md-header-anchor"></a><span>第二范式(2NF)</span></h3><p><span>第二范式要同时满足下面两个条件</span></p><blockquote><ul><li><strong><span>满足第一范式</span></strong></li><li><strong><span>没有部分依赖</span></strong></li></ul></blockquote><p><span>例如，员工表的一个候选键是{id，mobile，deptNo}，而deptName依赖于deptNo，同样 name 依赖于 id，因此不是 2NF的。为了满足第二范式的条件，需要将这个表拆分成employee、dept、employee_dept、employee_mobile四个表。如下：</span></p><p><strong><span>员工表 employee</span></strong></p><figure><table><thead><tr><th><span>id</span></th><th><span>name</span></th><th><span>zip</span></th><th><span>province</span></th><th><span>city</span></th><th><span>district</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>张三</span></td><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td></tr><tr><td><span>102</span></td><td><span>李四</span></td><td><span>200001</span></td><td><span>上海</span></td><td><span>上海</span></td><td><span>静安区</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>510001</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td></tr></tbody></table></figure><p><strong><span>部门表 dept</span></strong></p><figure><table><thead><tr><th><span>deptNo</span></th><th><span>deptName</span></th></tr></thead><tbody><tr><td><span>D1</span></td><td><span>部门1</span></td></tr><tr><td><span>D2</span></td><td><span>部门2</span></td></tr><tr><td><span>D3</span></td><td><span>部门3</span></td></tr><tr><td><span>D4</span></td><td><span>部门4</span></td></tr><tr><td><span>D5</span></td><td><span>部门5</span></td></tr></tbody></table></figure><p><strong><span>员工部门关系表 employee_dept</span></strong></p><figure><table><thead><tr><th><span>id</span></th><th><span>deptNo</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>D1</span></td></tr><tr><td><span>101</span></td><td><span>D2</span></td></tr><tr><td><span>102</span></td><td><span>D3</span></td></tr><tr><td><span>103</span></td><td><span>D4</span></td></tr><tr><td><span>104</span></td><td><span>D5</span></td></tr></tbody></table></figure><p><strong><span>员工电话表 employee_mobile</span></strong></p><figure><table><thead><tr><th><span>id</span></th><th><span>mobile</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>13910000001</span></td></tr><tr><td><span>101</span></td><td><span>13910000002</span></td></tr><tr><td><span>102</span></td><td><span>13910000003</span></td></tr><tr><td><span>103</span></td><td><span>13910000004</span></td></tr></tbody></table></figure><h3><a name="第三范式3nf" class="md-header-anchor"></a><span>第三范式(3NF)</span></h3><p><span>第三范式要同时满足下面两个条件</span></p><blockquote><ul><li><strong><span>满足第二范式</span></strong></li><li><strong><span>没有传递依赖</span></strong></li></ul></blockquote><p><span>例如，员工表的province、city、district依赖于zip，而zip依赖于id，换句话说，province、city、district传递依赖于id，违反了 3NF 规则。为了满足第三范式的条件，可以将这个表拆分成employee和zip两个表，如下</span></p><p><strong><span>employee</span></strong></p><figure><table><thead><tr><th><span>id</span></th><th><span>name</span></th><th><span>zip</span></th></tr></thead><tbody><tr><td><span>101</span></td><td><span>张三</span></td><td><span>100001</span></td></tr><tr><td><span>102</span></td><td><span>李四</span></td><td><span>200001</span></td></tr><tr><td><span>103</span></td><td><span>王五</span></td><td><span>510001</span></td></tr></tbody></table></figure><p><strong><span>地区表area</span></strong></p><figure><table><thead><tr><th><span>zip</span></th><th><span>province</span></th><th><span>city</span></th><th><span>district</span></th></tr></thead><tbody><tr><td><span>100001</span></td><td><span>北京</span></td><td><span>北京</span></td><td><span>海淀区</span></td></tr><tr><td><span>200001</span></td><td><span>上海</span></td><td><span>上海</span></td><td><span>静安区</span></td></tr><tr><td><span>51000</span></td><td><span>广东省</span></td><td><span>广州</span></td><td><span>白云区</span></td></tr></tbody></table></figure><p><span>在关系数据库模型设计中，一般需要满足第三范式的要求。如果一个表具有良好的主外键设计，就应该是满足3NF的表。规范化带来的好处是通过减少数据冗余提高更新数据的效率，同时保证数据完整性。然而，我们在实际应用中也要防止过度规范化的问题。规范化程度越高，划分的表就越多，在查询数据时越有可能使用表连接操作。而如果连接的表过多，会影响查询性能。关键的问题是要依据业务需求，仔细权衡数据查询和数据更新关系，指定最合适的规范化程度。不要为了遵循严格的规范化规则而修改业务需求</span></p><h2><a name="数据库一对一一对多多对多设计" class="md-header-anchor"></a><span>数据库一对一、一对多、多对多设计</span></h2><hr /><p><span>数据库实体间有三种对应关系：一对一、一对多、多对多</span></p><p><span>一对一关系示例：</span></p><p><span>一个学生对应一个学生档案材料 每个人都有唯一的身份证号</span></p><p><span>一对多关系示例：</span></p><p><span>一个学生只属于一个班，但这个班有多名学生</span></p><p><span>多对多关系示例：</span></p><p><span>一个学生可以选择多门课，一门课也可以有多名学生</span></p><p><span>一个人可以有多个角色，一个角色可以有多个人</span></p><p>&nbsp;</p><p><span>一、一对多关系处理</span></p><p><img src="https:////upload-images.jianshu.io/upload_images/3781695-ad93b299b2fc9c2a.png?imageMogr2/auto-orient/strip|imageView2/2/w/517/format/webp" referrerpolicy="no-referrer" alt="img"></p><p><span>设计数据库表：只需在 学生表 中多添加一个班级号的ID即可</span></p><p><span>二、多对多关系处理</span></p><p><img src="https:////upload-images.jianshu.io/upload_images/3781695-5dbc50523244885e.png?imageMogr2/auto-orient/strip|imageView2/2/w/866/format/webp" referrerpolicy="no-referrer" alt="img"></p><p><img src="https:////upload-images.jianshu.io/upload_images/3781695-9ff016c8cea50772.png?imageMogr2/auto-orient/strip|imageView2/2/w/1200/format/webp" referrerpolicy="no-referrer" alt="img"></p><p>&nbsp;</p><h1><a name="关系" class="md-header-anchor"></a><span>关系</span></h1><ul><li><p><span>创建成绩表scores，结构如下</span></p><ul><li><span>id</span></li><li><span>学生</span></li><li><span>科目</span></li><li><span>成绩</span></li></ul></li><li><p>&nbsp;</p></li><li><p><span>思考：学生列应该存什么信息呢？</span></p></li><li><p><span>答：学生列的数据不是在这里新建的，而应该从学生表引用过来，关系也是一条数据；根据范式要求应该存储学生的编号，而不是学生的姓名等其它信息</span></p></li><li><p><span>同理，科目表也是关系列，引用科目表中的数据</span></p></li></ul><p><img src="images/r.png" referrerpolicy="no-referrer" alt="关系"></p><ul><li><span>创建表的语句如下</span></li></ul><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded md-focus" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap CodeMirror-focused" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 181.497px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">create table scores(</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">id int primary key auto_increment,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">stuid int,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">subid int,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">score decimal(5,2)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 135px;"></div><div class="CodeMirror-gutters" style="display: none; height: 135px;"></div></div></div></pre><h1><a name="外键" class="md-header-anchor"></a><span>外键</span></h1><ul><li><span>思考：怎么保证关系列数据的有效性呢？任何整数都可以吗？</span></li><li><span>答：必须是学生表中id列存在的数据，可以通过外键约束进行数据的有效性验证</span></li><li><span>为stuid添加外键约束</span></li></ul><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">alter table scores add constraint stu_sco foreign key(stuid) references students(id);</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><ul><li><span>此时插入或者修改数据时，如果stuid的值在students表中不存在则会报错</span></li><li><span>在创建表时可以直接创建约束</span></li></ul><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">create table scores(</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">id int primary key auto_increment,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">stuid int,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">subid int,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">score decimal(5,2),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">foreign key(stuid) references students(id),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">foreign key(subid) references subjects(id)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 180px;"></div><div class="CodeMirror-gutters" style="display: none; height: 180px;"></div></div></div></pre><h4><a name="外键的级联操作" class="md-header-anchor"></a><span>外键的级联操作</span></h4><ul><li><span>在删除students表的数据时，如果这个id值在scores中已经存在，则会抛异常</span></li><li><span>推荐使用逻辑删除，还可以解决这个问题</span></li><li><span>可以创建表时指定级联操作，也可以在创建表后再修改外键的级联操作</span></li><li><span>语法</span></li></ul><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">alter table scores add constraint stu_sco foreign key(stuid) references students(id) on delete cascade;</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 45px;"></div><div class="CodeMirror-gutters" style="display: none; height: 45px;"></div></div></div></pre><ul><li><p><span>级联操作的类型包括：</span></p><ul><li><span>restrict（限制）：默认值，抛异常</span></li><li><span>cascade（级联）：如果主表的记录删掉，则从表中相关联的记录都将被删除</span></li><li><span>set null：将外键设置为空</span></li><li><span>no action：什么都不做</span></li></ul></li></ul></div>
</body>
</html>